Don't deliver email if the address is on a blacklist

unsubscribe.rb

class EmailAddressFilter
  def self.delivering_email(message)
    message.perform_deliveries = false

    # Remove email from message.to if email is on the unsubscribe list
    tos = message.to.to_a
    message.to.each do |email|
      tos.delete(email) if Unsubscribe.find_by_email(email)
    end
    message.to = tos # can't delete directly from message.to. probably a better way to do this...

    return unless message.to.any?

    # Still have emails? Let's send them!
    message.perform_deliveries = true
  end
end

ActionMailer::Base.register_interceptor(EmailAddressFilter)

How about

message.to = message.to.select { |email| !Unsubscribe.find_by_email(email) }